Vitrification holds promise as an approach to disposing of hazardous wastes and recycling glass manufacturing scrap. In the manufacture of glass, vitrifiable material is heated to its melting point and then cooled to form glass.
Conventional glass-making furnaces are large refractory lined tanks using direct heat from gas burned in the furnace above the upper surface of a pool of molten glass. Electric glass furnaces have also been developed which heat vitrifiable material by application of electrical energy through the material which is commonly referred to as Joule effect heating. Fluid flow in such furnace is primarily convective flow. In either gas fired or electrically heated glass-making furnaces, only limited agitation, if any, of the glass bath is permitted so as to minimize formation of bubbles in the glass. Bubbles are generally undesirable in finished glass products.
In the early 1970's, Owens-Illinois developed a three step rapid melting refining system described in U.S. Pat. Nos. 3,850,606; 3,654,886; 3,988,138 and 3,819,350. This rapid melting refining system project is summarized in an article entitled "Rapid Melting and Refining System", Ray S. Richards, Ceramic Bulletin, Volume 67, Number 11, 1988 Page 1806. In the rapid melting refining system, the glass-making process was divided into three separate steps. Special machinery was designed for heating glass to melt batch material in a first step, homogenizing glass to remove sand grains and seeds in a second step and refining the glass mix by the removal of seeds and bubbles by centrifuging in a third step.
The Owens-Illinois system was directed to the manufacture of glass used in making containers and the like. The rapid melting system achieves an equivalent melting capacity of a conventional cold top electric melter with only ten to fifteen per cent of the melt area required by the conventional melter. The smaller melter size and lower temperatures reduce volitization and heat losses. In addition, the size and cost of air pollution control equipment may be reduced. Lower average temperatures for maintaining a glass melt in molten condition of 100.degree. to 200.degree. F. are achieved because a uniform integral glass temperature is maintained. These advantages of the rapid melting process for recycling or vitrification were not recognized by the trade prior to this invention. The 1988 Richards article presented the results of the earlier work and proposed adaptation of the Owens-Illinois process for vitrification of hazardous waste material including low-level radioactive waste, municipal incinerator waste and asbestos waste material. However, an apparatus and method for implementing that proposed process had not been attempted and was not disclosed or suggested in the 1988 Richards article.
The Owens-Illinois system employed the impeller of the mixing device as a primary electrode for Joule-effect heating. Current concentration at the tips of the impeller and the use of single phase power limited scale up of the Owens-Illinois melter. The problems related to converting the melting and refining system proposed by Owens-Illinois to vitrification of hazardous waste material and recycling are addressed by the present invention. The present invention also addresses some of the unsolved problems which were encountered by the rapid melting and refining system referred to above.
The primary problems associated with conventional gas and Joule heated melters is their large size and cost, expensive air pollution control equipment, energy costs, their need for continuous uninterrupted production and their inability to change production rates significantly without quality upsets.
Another kind of problem encountered in conventional glass-making or vitrification processes can be categorized in part by reference to the feed stream supplied to the glass melting furnace. The conventional feed stream for the glass-making process includes "raw batch" and may also include cullet such as recycled bottles, glass beads, specialty glasses, window glass or glass foam. "Raw batch" may also include mixtures of silica, alkali and stabilizing ingredients such as lime, alumina, lead and barium. A primary problem associated with processing such feed streams is segregation and subsequent homogeneity control. Another problem with prior art systems is that pre-blended batch can only be pre-heated to a limited extent without it adhering to and blocking equipment.
It would be desirable to recycle scrap from mineral wool production. Fiberglass scrap may have up to fifteen per cent organic binder. When this scrap is added to conventional melters, carbon residue from organic binders in the scrap is trapped in the melt and creates an unacceptable black glass. Scrap mineral fibers are light-weight and tend to float on the surface of the glass melt where they obstruct heat transfer. Light-weight feed streams also can be carried out of the furnace in the exhaust gas stream.
Fly ash and bottom ash from incinerators generally referred to as ash, may include highly toxic material which can be made resistant to leaching by vitrification in a glass melting furnace. Fly ash presents problems which are in some respects similar to problems faced in recycling scrap mineral fibers, in that a light-weight, low density feed stream must be introduced into the glass melt. An additional problem relating to the vitrification of ash is that ash changes composition depending upon the source of ash and the constituents of the waste incinerated. For example, in the fall of the year a large volume of organic waste from leaf disposal is processed in municipal waste incinerators. This change in composition of the ash may require modification of the chemical constituents supplied to the glass melt in addition to the ash. Fly ash also presents special problems due to its toxicity which creates handling problems.
Radioactive wastes may be in the form of a liquid slurry or dry waste. Radioactive waste can further be divided into high-level, intermediate and low-level radioactive waste. Another radioactive waste stream is contaminated earth. A problem associated with vitrification of radioactive waste is handling radioactive material in a safe manner. Some radioactive waste streams include absorbent pads used to absorb minor spills of contaminated material at radioactive sites. One problem associated with the disposal of absorbent pads is the large volume of organic material used to absorb a small amount of radioactive material which exacerbates waste disposal problems.
Industrial waste feed streams including those from plating, painting and other industrial waste present special waste disposal problems which can be addressed by vitrification. Toxic inorganic substances found in chemical and industrial waste streams may be disposed of with excellent leach resistance when vitrified. Problems relating to chemical industry waste include disposal of incinerator bottom and fly ash. Bottom ash may include a considerable volume of metal which can interfere with Joule effect heating.
It is anticipated that other feed streams, including but not limited to asbestos and refractory fibers, may be processed by vitrification and problems associated therewith may be solved by applicant's invention as summarized below.